What type of insulation are you guys using when sleeping in an igloo? I'm specifically looking for thoughts on sleeping bag insulation. I have a MLD bivy on order that looks like it won't be here in time, and now I'm worried about taking my down bag (15 degree) into such a wet environment. I have a synthetic bag that I could use, but it's rated much colder (minus 20 degree) and is much heavier than I'll need. Temps should be in the teens at worst, +/- wind. We'll be bringing an alternate shelter to be used if igloo building conditions aren't favorable, and we'll only be about a 3 hour hike from the car, so this isn't a life or death decision. But assuming we're successful in building an igloo, I don't want to have to bail just because my bag is saturated. I'll be sleeping on a regular length therm-a-rest x-therm.

I've never slept in an igloo either. Should be a fun learning experience. I've only snow camped in tents. I'm hoping the MLD bivy will help expand my options to tarps and/or floorless mids when the weather or exposure doesn't require a full tent. I have no idea how wet of an environment an igloo is, but I hear it's wet. We'll do what we can to divert dripping water, but we're new at this so I don't know how successful we'll be. Been reading a ton on construction. No snow close enough to practice. We'll be heading to the south side of Mt. St. Helens where I've tent camped so I know the area.

With a down bag in a tunnel or hole, what are your tricks for keeping things dry?

What I recommend is to use a down bag or synthetic bag over a normal sleeping pad. There are ways of dealing with the moisture. First of all, excess moisture tends to form at the center of the ceiling, because there is where the heat rises. If the ceiling is less than perfectly smooth, then moisture may drip off of those little points. If the ceiling is smooth enough, then the moisture starts to migrate toward the side walls and then move down. If your sleeping bag is laying up against the igloo wall when this moisture is moving, you will have a problem. Instead a simple plastic tarp or mylar space blanket can be placed, perhaps half under the sleeping bag and half against the igloo wall. Then, if the moisture is moving, it will go behind the tarp. If you leave the igloo, you can fold the tarp more directly over the sleeping bag just so nothing will drip on it.

When the igloo is constructed, it is nice to have sleeping benches made out of snow or ice. These will raise the sleeping bag off the floor by a foot or so. That is recommended since the coldest air will sink to the lowest point.

Normally you want to punch an air hole in the ceiling of the igloo, and then have another air vent somewhere low like at the door. That way, there will be a slight flow of air which will also decrease the humidity.

A lot depends on where this will be. California snow is a lot different from Colorado snow which is a lot different from Maine snow, etc.

Since we've had a very sparse snow year in the PNW, conditions are warm and wet in the day and windy and icy at night (at elevation, that is, there's no snow down low). I expect it will be a wet night. I have some extra ground sheet material that I can use as a wall and splash guard with minimal effort or weight. I imagine having it between the bag and the wall is a good idea as well as draping a little over the top of the bag. I'll bring a trash compactor bag and my rain shell too, so I'll have that to work with. It will still be lighter than the heavier synthetic bag alone.

Seems like most people still bring down into igloos, so I just need to work on the techniques required to stay dry.